Page 7 - i1052-5173-32-7

Lodgepole lithostratigraphic descriptions by downlap and stratal thinning. The basinal
are from locations in Figures 6 and 7. Log- facies consists of gray to black, generally
defined units described in Manitoba by massive, locally laminated, moderately high
Stanton (1958) and carried through North gamma-ray (100–150 API units), organic-
Dakota by Grover (1996) are used within rich, sparsely skeletal, argillaceous (39%
the Lodgepole Formation. The lower Virden clay), calcareous (37% calcite), low-pyrite
subinterval is defined by a higher gamma- (1%), silty mudstone. TOC contents vary
ray response than the underlying Scallion from 1.1%–5.3% for three samples from
and overlying upper Virden subintervals wells in Figure 6, to 5%–8% for mature sam-
(Fig. 6). Grover (1996) defined a clinoform ples in Price and LeFever (1994), equivalent
geometry that was correlated throughout to 7%–12% in immature samples.
North Dakota where a thin (5–15 m) land-
ward shelf facies transitions seaward to a BAKKEN BIOSTRATIGRAPHIC
thick (10–80 m) slope facies, which transi- UNITS
tions seaward to a thin (1–10 m) basinal Bakken biostratigraphy studies define
facies, as depicted in Figure 6. The lower stratigraphic units characterized by lithol-
Lodgepole clinoform geometry is illustrated ogy, paleobiology, and chemostratigraphy.
in Grover (1996, figs. 14, 16, 18, 20, 22, and All of these are described as “biostrati-
33), Skinner et al. (2015, slide 12), Petty graphic units” in Figure 2. The Lower
(2019a, figs. 10 and 11) and Figure 6 (this Member of the Bakken Formation was sub-
paper). In the Black Hills and the type area divided into four lithostratigraphic-bio-
of Virden Field (Fig. 7), the shelf facies is stratigraphic-chemostratigraphic units in
characterized by skeletal-oolitic limestone, North Dakota by Hogancamp and Pocknall
microbial-peloidal-intraclastic packstone, (2018); these were designated LB1, LB2,
argillaceous mudstone, laminated micro- LB3, and LB4. These units display a regional,
crystalline dolostone, and thin stratiform landward, intra-shale onlap pattern in a
breccias that represent former evaporite northeast-southwest profile (Hogancamp
beds. The slope facies consists of brown to and Pocknall, 2018, fig. 4); the LB4 unit is
gray, massive to faintly laminated, argilla- present locally. The Middle Member litho-
ceous, medium gamma-ray (40–100 API stratigraphic units of this study (MMA,
units), organic-lean (0.4% TOC), silty, skel- MMB, MMC, and MMD) either correspond
etal-peloidal, calcareous mudstone to pack- with, or parallel, the Middle Member bio-
stone. The slope facies transitions seaward stratigraphic units of Holland et al. (1987),
to the basinal facies that is confined to the Thrasher (1987), and Hogancamp and
center of the Williston basin (Fig. 7). This Pocknall (2018), as depicted in Figures 2
facies thins from 10 m or less landward to and 5. The Upper Member of the Bakken
1 m or less in the basin-center. Basinward Formation was subdivided into two lithostrati-
Figure 5. Type logs with gamma-ray character for thinning within the lower Virden is caused graphic-biostratigraphic-chemostratigraphic
lithostratigraphic units of this paper (right) and
Bakken biostratigraphic units (left) of Hogan-
camp and Pocknall (2018, fig. 5). Logs represent
the most-seaward and most-landward cored
wells in Figure 3. Note that logs have different
vertical scales (meters) and horizontal scales
(API units). API—American Petroleum Institute.

presence of oolitic grainstone, skeletal-oolitic
grainstone, sandy-oolitic grainstone, sand-
stone, and prominent cross stratification
makes the MMC the most distinctive Middle
Member unit. The Middle Member D (MMD)
lithostratigraphic unit consists of argilla-
ceous, dolomitic siltstone. The Upper Member
consists dominantly of black, faintly lami-
nated to massive, organic-rich (2%–26%
TOC from LeFever, 2008), high gamma-ray
(150–1100 API units), argillaceous (30% clay
minerals), pyritic (6%), low-calcite (2%),
silty shale that displays a layer-cake geom- Figure 6. Cross section C–C′ showing lower Lodgepole stratigraphic relationships. Cross section loca-
etry regionally (Fig. 3). tion in Figure 7. API—American Petroleum Institute; GR—gamma ray.

www.geosociety.org/gsatoday 7